Thus it is also conventional, based on the composition of the exhaust gas, to deduce the storage capacity. For this purpose a lambda probe is provided which is located downstream from the oxygen storage reservoir, typically the entire catalytic converter, in the flow direction of the exhaust gas, or which is located in the oxygen storage reservoir and hence is located downstream from part of the oxygen storage reservoir. In this instance, the measured values of the lambda probe are the values of the voltage which is on the lambda probe. The storage capacity of the oxygen storage reservoir is reflected in the time behavior of the measured values when a change has taken place in the loading of the oxygen storage reservoir.
The oxygen storage reservoir does not work as long as it is supplied with exhaust gas with an air-fuel ratio of λ=1, since then the catalytic converter which has storage capacity by means of oxygen storage reservoir units causes all the fuel to be burned without oxygen residue. If the oxygen storage reservoir is supplied with rich exhaust gas, that is, with exhaust gas in which the proportion of fuel is higher, oxygen is removed from the oxygen storage reservoir. First of all, this is not dramatically reflected in the lambda probe located downstream from the oxygen storage reservoir, but there is a voltage jump as soon as the oxygen storage reservoir has been emptied, since then exhaust gas arrives at this lambda probe. It is the reverse when the oxygen storage reservoir is supplied with lean exhaust gas, that is, with exhaust gas in which the air-fuel ratio λ is greater than 1, that is, there is more air than is required to react the fuel. Then atmospheric oxygen is removed and stored in the oxygen storage reservoir. Only when the oxygen storage reservoir is filled is there a jump in the measured values, because lean exhaust gas can not further release oxygen and arrives at the lambda probe.
At this point the situation is such that the oxygen storage reservoir can lose storage capacity especially due to ageing or poisoning with contaminants, but in the exact same way the lambda probe can also have undesirable behavior due to ageing or poisoning. It has been shown that the state in which the storage capacity of the oxygen storage reservoir is sufficient, but at the same time the lambda probe is not serviceable, can only be distinguished with difficulty from the state in which the storage capacity of the catalytic converter is not sufficient, with a fully serviceable lambda probe.
DE 10 2005 016 075 B4 describes a method for diagnosis of a lambda probe assigned to the exhaust gas catalytic converter of an internal combustion engine, in which the time delay in the jump of the signal of a lambda probe located upstream from the catalytic converter to the lambda probe located downstream from the exhaust gas catalytic converter is detected and specifically for two different mass flows of exhaust gas. Then it is determined from the measured time delays how large the catalytic converter-dictated proportion which is dependent on the exhaust gas mass flow is in the time delays and how large the probe-dictated proportion which is essentially independent of the exhaust gas mass flow is in the time delays. Only the probe-dictated proportion is used as the diagnosis criterion for the lambda probe.
In a refinement of the method described in DE 10 2005 016 075 B4, in DE 10 2008 023 893 which was published after the application date of this application, it is described that lean exhaust gas and rich exhaust gas are supplied to the catalytic converter in an alternating sequence into the exhaust gas line and a signal recorded when changing from lean to rich exhaust gas or vice versa from the lambda probe located downstream from the catalytic converter is used for diagnosis. This is based on the finding that the exhaust gas and its composition change slightly in such a change, even if the actual jump in the measured values of the lambda probe takes place only when after longer supply of rich exhaust gas the oxygen storage reservoir is emptied or after longer supply of lean exhaust gas the oxygen storage reservoir is completely filled.
The object of the invention is to show how four situations can be separated from one another by an analysis process, specifically that a) both the probe and also the oxygen storage reservoir are still operating well enough, b) that neither the probe and oxygen storage reservoir are well enough, or that c) the probe is working well enough, the oxygen storage reservoir, however, no longer has sufficient oxygen storage capacity, or that d) the oxygen storage reservoir has sufficient storage capacity, the probe, however, is no longer working well enough.